Stirlingengineforum.com

I do not understand why there is a big debate of whether the engines we work on here are described as having either isothermal or adiabatic expansion and compression. So which is it?

Simple answer, neither. The simplest answer is polytropic, however that is fairly useless. It boils it down to all expansions are polytropic in the real world.

Some would contend that it is more Adiabatic than isothermal. That might be very accurate for some, or many, engines. It might not be for engines that run very slowly and have short distances between displacers and flat plate heat exchangers, such as a typical LTD Stirling Engine.
From those three charts it can be seen that a typical Stirling indicator neither is close to isothermal (thin isotherms) nor adiabatic (thick green line). The curving of the indicator diagram is caused mostly by the harmonic motion of the displacer and power pistons, and their 90° separation.

At times the indicator diagram appears close to an isotherm. At other times it is quite far away. Other engines may be closer to the Adiabatic line.

It can also be noted that if an engine were to follow the adiabatic, thick green line, for both expansion and compression, they would negate each other and zero work would result. Adiabatic means zero heat transfer. Zero heat in and out means zero work. Seems obvious when looking at the chart.

The best that could be done would be to follow the isotherms. Hot during expansion, cold during compression. That is very unlikely to happen without both pistons being delayed at the end of their strokes.

The power generated is directly related to the area inside the indicator diagram. Indicator diagrams are generated by measuring pressure and volume of real engines. Volume is measured by crank angle. Pressure is measured by a gauge.

The expansion and compression lines will always be between the adiabatic lines and the isotherms.

P.S. It is colloquial to say any fast expansion is adiabatic, even when is is not. That is where the confusion may lie.

Nobody wrote: ↑Sat Dec 04, 2021 9:26 am...



. Adiabatic means zero heat transfer. Zero heat in and out means zero work. Seems obvious when looking at the chart....

Yep. The work done in one direction equals the work absorbed in the opposite direction. Zero net work for the depicted cycle.

Nobody wrote: ↑Sun Dec 05, 2021 10:52 am Yep. The work done in one direction equals the work absorbed in the opposite direction. Zero net work for the depicted cycle.

I think that is a miss-interpretation

The heat added at the hot side causes the gas to expand and do work, but because the gas expands adiabatically (mostly, or partly, anyway, nothing is 100%) and the energy in the gas goes out as work, the gas cools and the inner pressure drops below the external atmospheric pressure (or buffer pressure in a pressurized engine).

So, the piston returns due to the outside atmospheric pressure This is positive work, contributing to the total work output, not subtracting from it. What Senft calls "constant mechanical effectiveness".

Because the inner pressure drops below the external 1atm in an open cylinder, or below the buffer pressure in a pressurized engine, the compression work is "effective" work not "forced" work (to use Senft's terminology). In this way atmospheric or buffer pressure also contributes "positive" work to the cycle.

There is also not just one adiabat.

At TDC, the top of the compression stroke, there is (mostly) isochoric heat addition for the next cycle, raising the potential for doing work for another cycle.

In other words, the adiabatic portions of the compression and expansion are not necessarily at the same temperature, not the same adiabat. (Though I don't think it really matters, either way it's positive ("effective") work being done in both directions)

There is positive work being done in both directions, first expansion by heat addition, then compression by atmospheric pressure, (or buffer pressure).

So instead of work done in one direction and work "absorbed" in the other, it is work done in both.

Expansion work by the inner gas and compression by atmosphere (or buffer).

There is so much wrong in the last post that it is difficult to find ground to reply. Let me try just the stuff you got correct.

In other words, the adiabatic portions of the compression and expansion are not necessarily at the same temperature, not the same adiabat. (Though I don't think it really matters, either way it's positive ("effective") work being done in both directions)

Your description of the process is close to a Carnot Engine. Two different adiabatic lines, separated by heat addition at the top, constant volume, part of the stroke, and heat removal at the bottom part of the stroke. Then zero heat transfer during the entire stroke in both directions. (That is not what the single adiabatic line above is depicting. But let's go with it for the moment.)

Yes heat is added and heat is rejected, to obtain two different adiabatic lines. Without both, added and rejected heat, the lines won't be separated. (That is accomplished in an ideal Carnot cycle by assuming the piston stops and delays at TDC and BDC.) Merely adding heat at TDC won't do it. Heat must be rejected to a cold sink. That is the reason for a Delta T. To add and remove heat, for two separate lines.

The regenerator will become useless because the reduction of working fluid
pressure and temperature to a point below the "buffer"/ambient pressure will be below ambiant temperature in the working fluid. There will be no Delta T, nothing to be absorbed, leading you back up the same adiabatic line. In other words, at the cold end there will be zero Delta T to move to the lower adiabatic line.

So a hot and cold heat exchange will be needed. And the expansion will need to stop before it gets to the cold sink temperature, so it can remove heat, so the return adiabatic line will be lower.

That is why a Carnot Engine is described without a regenerator. *

* Maybe that is why an acustical free piston (laminar flow) engine appears not to care about the ss-wool regenerator? It is running on a Carnot cycle approximation? I'm very skeptical here. Would need to analyze indicator diagrams.

Because the inner pressure drops below the external 1atm in an open cylinder, or below the buffer pressure in a pressurized engine, the compression work is "effective" work not "forced" work (to use Senft's terminology). In this way atmospheric or buffer pressure also contributes "positive" work to the cycle.

You are describing an engine that has an inside working fluid pressure, and outside ambient, or pressurized crankcase, pressure on the other side of the piston.

That turns each stroke into two phases. One when inside pressure is greater than the outside pressure, and the second when they are reversed.

One phase will produce positive work, while the other negative work. For each stroke.

Example: Power stroke. Start hot and high pressure, TDC. Piston starts moving, accelerating, positive work. Piston passes equal pressure point. Outside higher pressure than inside. Piston slows to a stop, negative work.

The same happens for the reverse compression stroke. Piston accelerates, positive work. Equal pressure point. Piston decelerates, compressing inside, negative work.

The above description is what the two shaded areas are in the Senft diagram.

If it helps, try to build, or visualize, an engine like that running in space or a large vacuum chamber. It is the same as having zero buffer pressure. You will need to restrain the piston with a spring or other device.